A number of conditions and diseases of the central (brain and spinal cord) and peripheral nervous system adversely affect humans. These conditions and diseases include, for example, spinal cord injury, amyotrophic lateral sclerosis (ALS), Parkinson's disease, stroke, traumatic brain injury, brain tumors, multiple sclerosis (MS), and Fabry Disease. Clinical management strategies frequently focus on the prevention of further neurological damage or injury rather than replacement or repair of the damaged neurological tissue (e.g., neurons, glial cells). These strategies can include treatment with exogenous steroids and synthetic, non-cellular pharmaceutical drugs and can have varying degrees of success, which may depend on the continued administration of the steroid or synthetic drug.
For this reason, there is a great deal of evolving interest in neural progenitor cells. Up until the present time, it was generally thought that multipotent neural progenitor cells commit early in the differentiation pathway to either neural restricted cells or glial restricted cells. These in turn are thought to give rise to mature neurons, or to mature astrocytes and oligodendrocytes. Multipotent neural progenitor cells in the neural crest also differentiate to neurons, smooth muscle, and Schwann cells. It is hypothesized that various lineage-restricted precursor cells renew themselves and reside in selected sites of the central nervous system, such as the spinal chord. Cell lineage in the developing neural tube has been reviewed in the research literature by Kalyani et al. (Biochem. Cell Biol. 6:1051, 1998).
Putative multipotent neuroepithelial cells (NEP cells) have been identified in the developing spinal cord. Kalyani et al. (Dev. Biol. 186:202, 1997) reported NEP cells in the rat. Mujtaba et al. (Dev. Biol. 214:113, 1999) reported NEP cells in the mouse. Differentiation of NEP cells is thought to result in formation of restricted precursor cells having characteristic surface markers.
Putative neural restricted precursors (NRP) were characterized by Mayer-Proschel et al. (Neuron 19:773, 1997). These cells express cell-surface PS-NCAM, a polysialylated isoform of the neural cell adhesion molecule. They reportedly have the capacity to generate various types of neurons, but do not form glial cells.
Putative glial restricted precursors (GRPs) were identified by Rao et al. (Dev. Biol. 188: 48, 1997). These cells apparently have the capacity to form glial cells but not neurons.
Ling et al. (Exp. Neurol. 149:411, 1998) isolated progenitor cells from the germinal region of rat fetal mesencephalon. The cells were grown in EGF, and plated on poly-lysine coated plates, whereupon they formed neurons and glia, with occasional tyrosine hydroxylase positive (dopaminergic) cells, enhanced by including IL-1, IL-11, LIF, and GDNF in the culture medium.
Wagner et al. (Nature Biotechnol. 17:653, 1999) reported cells with a ventral mesencephalic dopaminergic phenotype induced from an immortalized multipotent neural stem cell line. The cells were transfected with a Nurr1 expression vector, and then cocultured with VM type 1 astrocytes. Over 80% of the cells obtained were claimed to have a phenotype resembling endogenous dopaminergic neurons.
Mujtaba et al. (supra) reported isolation of NRP and GRP cells from mouse embryonic stem (mES) cells. The NRPB were PS-NCAM immunoreactive, underwent self-renewal in defined medium, and differentiated into multiple neuronal phenotypes. They apparently did not form glial cells. The GRPs were A2B5-immunoreactive, and reportedly differentiated into astrocytes and oligodendrocytes, but not neurons.